1. Field of the Invention
The present invention relates to a chain tensioner used to apply an appropriate tensional force to a transmission belt or a transmission chain in the timing system of an engine or the like.
2. Description of the Related Art
A known chain transmission device used in the timing system of an engine or the like generally employs, in order to apply an appropriate tensional force to the slacked side of a transmission belt or a transmission chain via a tensional lever to reduce vibration occurring at running, a chain tensioner having a tensioner body that has a plunger accommodation hole which is open on one side, a plunger slidably inserted in the plunger accommodation hole, and an urging mechanism that urges the plunger in a protruding direction with a spring or the like.
In order to restrict the movement of a plunger in an accommodation portion to prevent its fluttering due to a great change in tensional force at starting or the like and temporarily stop the protrusion of the plunger at assembling, maintenance, or the like, a conventional chain tensioner 500 is provided with, in a tensioner body 510, an oscillation cam 530 having two ratchet pawls 531 capable of engaging with rack teeth 522 formed on the side surface of a plunger 520 as shown FIG. 16 (see, for example, Japanese Patent Application Laid-open No. 2006-234088, Japanese Patent Application Laid-open No. 2004-44749, and Japanese Patent Application Laid-open No. H11-344036).
In the chain tensioner 500, the rack teeth 522 of the plunger 520 and the ratchet pawls 531 of the oscillation cam 530 engage with each other to cause a mechanical backstop to reduce noise at starting until engine oil is supplied to the chain tensioner when an engine starts, and a certain backlash is set at the same engagement position to absorb the thermal expansion of the engine and a change in the elastic elongation of a chain.
In addition, in order to deal with the elongation of the chain over a long term, the chain tensioner 500 has a mechanism in which the plunger 520 advances step by step with a change in the engagement position between the rack teeth 522 of the plunger 520 and the ratchet pawls 531 of the oscillation cam 530.